Since an optical microscope, which is used to observe an organism, a microelement structure in a nanometer unit, or the shape of a surface, observes an object by using light, the resolution is limited due to a diffraction limit phenomenon. In other words, an object whose size is equal to or lower than ½ of the wavelength of the light cannot be optically observed. Accordingly, a near field optical microscope is developed, which can overcome such a diffraction limit and perform optical measurement at a much lower level than the wavelength of the light. In the near field optical microscope, light that passed through an opening smaller than the wavelength of the light irradiates an examined material that is at a distance similar to the size of the opening. Here, a near field that is at a smaller distance than the wavelength of the light from the surface of the examined material does not generate diffraction. Accordingly, in order to improve the resolution of the near field optical microscope, the size of the opening should be reduced and the distance between the opening and the surface of the examined material should be reduced.
A well known near field optical probe used in such a near field optical microscope is an optical fiber near field optical probe 100 as illustrated in FIG. 1. The optical fiber near field optical probe 100 thinly extends an optical fiber 102 by applying heat, or etches the optical fiber 102 by using a chemical so that the size of one end of the optical fiber 102 of the optical fiber 102 becomes several tens through several hundreds of nanometers. Then, a metal layer 104 is deposited on the optical fiber 102 in order to prevent light from escaping from the external surface of the optical fiber 102, and an opening 105, which has a diameter of several tens through several hundreds of nanometers, is formed at the end of the optical fiber 102. A reference numeral 103 denotes a near field.
In order to measure an optical characteristic of an examined material 106 having a nanostructure by using the optical fiber near field optical probe 100, the optical fiber near field optical probe 100 is drawn near to the examined material 106 in a range of several to several tens of nanometers. Then, an optical signal reflected from each irradiation point is measured while irradiating light onto the surface of the examined material 106, and an entire image is obtained by combining the optical signals.
In order to draw the optical fiber near field optical probe 100 up to a nanometer distance of the examined material 106, the optical fiber near field optical probe 100 is attached to a crystal oscillator 110, and the crystal oscillator 110 is vibrated at a uniform frequency by using a piezo oscillator 113. Then, a vibration signal is applied to the piezo oscillator 113 by using a lock-in amplifier 115. A signal detected from the crystal oscillator 110 changes according to a distance between the optical fiber near field optical probe 100 and the examined material 106. Accordingly, the distance between the optical fiber near field optical probe 100 and the examined material 106 can be adjusted by detecting the signal.
The detected signal is provided to a piezo translator 120 as a feedback signal through the lock-in amplifier 115 and a proportional integrator 117, and the moving amount of the piezo translator 120 is compensated by using the provided signal.
When the optical fiber near field optical probe 100 irradiates light onto the surface of the examined material 106, the detected signal of the crystal oscillator 110 changes according to the minute change of the surface of the examined material 106. Precise height information of the surface of the examined material 106 can be obtained by using such changes in the detected signal.